1. Field of the Invention
This invention pertains to a method for replicating (copying) a hologram into a photosensitive layer to produce a copy (replicate) of the hologram in a manner such that the copy is an accurate and true replication of the hologram (e.g., master hologram) with minimum defects.
2. Description of Related Art
Commercially acceptable holograms, used as security elements in graphic arts or as holographic optical elements, must be free of imaging defects. Holographic prior art teaches the importance and necessity of using optically clear, smooth materials which do not cause unwanted surface reflections during holographic processing, and especially during replication of a master hologram to produce a copy (replicate) of the master hologram. Replication by direct contact copying of a master hologram (either a reflection hologram or a transmission hologram) in which the master hologram is in direct contact with a photosensitive layer (e.g., a holographic recording film) is known from the art. With respect to reflection holograms, see, for example, the following references: 1) xe2x80x9cPhotographic Reconstruction of the Optical Properties of an Object in its Own Scattered Radiation Fieldxe2x80x9d, by Yu N. Denisyuk, Soviet Physicsxe2x80x94Doklady, 7, pages 543-5 (1962); 2) xe2x80x9cCopying Reflection Hologramsxe2x80x9d, by Clark N. Kurtz, Journal of the Optical Society of America, 58, pages 856-7 (1968); and U.S. Pat. No. 4,995,685. With respect to transmission holograms, see, for example, U.S. Pat. No. 4,209,250, which discloses a system for making multiple copies from a stationary planar transmission master hologram, and U.S. Pat. No. 4,973,113, which describes a method and apparatus for making a copy of a transmission hologram from a master. The prior art teaches that such direct contact copying is done by contacting a photosensitive element, comprised of a photosensitive layer and a smooth coversheet, to a smooth master hologram such that a smooth surface of the coversheet is in direct contact with a smooth surface of the master hologram. Frequently, contacting of the two smooth surfaces is effected with use of a vacuum platen that draws the coversheet in close contact with the master hologram. Contact copies made using the above prior art process frequently exhibit a defect known as xe2x80x9cNewton""s ringxe2x80x9d defects, which are circular rings of interference patterns of light and dark areas that appear together with an image of the master hologram in the copied hologram. The present invention provides a contact copying process for accurately reproducing a hologram which is devoid of xe2x80x9cNewton""s ringxe2x80x9d defects and other interference-related defects.
A method for copying (replicating) a hologram comprises:
(a) placing a photosensitive element in contact with an outer surface of a holographic element containing a master hologram; and
(b) exposing the photosensitive element and the holographic element to a coherent light beam whereby a replicate of the hologram is formed in the photosensitive element;
characterized in that the outer surface of the holographic element has a matte surface with a surface roughness measured by profilometry in the range from 5 to 200 nanometers and has a specular reflectance of at least five percent.
The present method for holographic replication involves using a holographic element (as holographic master) that has an outer surface that is matte, having a surface roughness measured by profilometry in the range from 5 nanometers to about 200 nanometers. The holographic element comprises an optically transparent layer disposed directly adjacent the master hologram, wherein the surface of the optically transparent layer opposite the master hologram comprises the outer surface of the element and has a specular reflectance of at least five percent. The outer surface can be inherently matte or it can be made to be matte by applying a matting agent (e.g., in a spray, in a coating, etc.).